This research note is a chapter in "Investigating Water with Seeds"
We attempt here to describe a conceptual framework that is understandable to anyone wishing to become involved in the investigation and analysis of water. For in situ work and also for "laboratory" work.
"Natural" water quality studies involve three steps:
Sampling is essential because it conditions the relevance of the analysis. It must be of good quality but also representative of what we want to analyze.
The water samples must be taken in clean containers, rinsed several times with the water to be analyzed, then hermetically closed without leaving air bubbles in the flask.
The nature of the material of the sampling container is important, because this one must not enter in reaction with the water to be analyzed => Passage in solution of chemical elements entering the composition of the bottle or fixing of certain ions of water on the walls of the container. The ideal material does not exist and the use of such a product rather than another depends on the nature of the elements to be measured:
Standard rectangular cuvettes (quartz glass or tetrafluoroethylene (Teflon)) is a suitable material for the good conservation of samples but proves to be very fragile (most commonly used type in chemical analysis)
glass and Pyrex can be used without problem for the determination of major ions. However, trace elements can pass in solution into the water (silica in glass and boron in Pyrex)
Polyethylene is often recommended for all sampling, especially for sampling for the measurement of radioactive elements. Relatively porous with respect to gases, glass is preferred for the sampling and conservation of water heavily charged with gas (diffusion is however rather slow)
Polyvinyl sampling bags have the advantage of being used only once, but they have the disadvantage of releasing chlorine if they are exposed to light for too long
General Precautions and Procedures
Rinse container 2-3 times with the sample to be collected
Fill the container full, leaving approximately 1% of the volume empty to allow for possible thermal expansion
if sampling a stream, collect the sample at mid-stream and at mid-depth
(Source: "Standard Methods for the Examination of Water and Wastewater", 19th Ed., 1995, A.D. Eaton, L.S. Clesceri, and A.E. Greenberg, ed.)
Possible modifications of the sample
Taking a water sample and separating it from its natural environment leads to more or less significant modifications depending on the parameters. Some of them can be considered as stable at the time scale at which we work, but others vary very quickly: temperature, conductivity, pH and dissolved gases, and nitrates and sulfates.
A variation in temperature leads to a modification of the equilibrium constants of the elements in suspension. In order to establish new equilibrium at the new ambient temperature, various chemical reactions occur which can lead to the precipitation of salts and the dissolution of gases. However, a low temperature (about 4°C) blocks the evolution of the reactions.
Contact with air and decompression are also responsible for changes within the solution.
Each ionic species contributes to the total conductivity of a solution. Therefore, any change in chemical equilibrium, and therefore in the relative proportions of dissolved elements, leads to a change in conductivity. The higher the temperature of the water, the more CO2 tends to escape from the solution. A departure of CO2 can cause the precipitation of carbonate, which in turn changes the pH. Nitrates and sulfates can be reduced by bacterial activity.
To simplify, we will say here that there are 2 main types of procedures (United States Environmental Protection Agency):
General Sampling Procedures
Specific Sampling Procedures for
- fibrous silicate mineral
- Biological Contaminants
- Classical Chemistry Constituents and Nutrients (IOCs)
- Colorimetric Analysis for Disinfectant Residuals
- Haloacetic Acids (HAA5s)
- Metals (IOCs) and Common Metal Contaminants
- Semivolatile Synthetic Organic Compounds (SOCs)
- Total Organic Carbon (TOC)
- Volatile Organic Compounds (VOCs)
General sampling procedures:
Prepare a Sampling and Analysis Plan (SAP) which describes the sampling locations, numbers and types of samples to be collected, and the quality control requirements of the project
Samples must be taken in such a way as to avoid edge effects as much as possible:
- Oxygenation when sampling too close to the surface,
- Suspension of solids when sampling too close to the bottom,
- stagnant water when sampling too close to the shoreline
It may be necessary to make up an "average" sample by mixing several samples taken at various points in a section of river, in order to gain a better understanding of the average water chemistry over a given section. On the other hand, it is essential to take different samples in space and time and treat them separately to study the functioning of a pond.
Wells and boreholes
A sample taken from water that has been stagnant for a long time is not representative of the water table. Indeed, the water has been influenced by the casing material and external elements (pollution, rain). To obtain an average sample of the captured horizon, it is necessary to pump long enough to renew the water contained in the casing. If the structure captures several aquifer horizons, the sample will be a mixture of the different waters, the proportions of which are directly related to the transmissivities of the different levels. To obtain spot samples at different depths, it is possible to use weighted containers equipped with a closing system that can be operated from the surface (hydrocatchers). To limit the mixing of the water during sampling, it is recommended to use hydrocaptors with a diameter much smaller than that of the borehole and to handle them extremely slowly. The use of hydrocaptors lowered in open position allows to rinse them as they are lowered.
In situ measurements
Temperature, pH, conductivity, alkalinity and dissolved oxygen must be measured in situ. Indeed, these parameters are very sensitive to environmental conditions and may vary in important proportions if they are not measured on site.
Ideally, measurements should be taken continuously (except for alkalinity, which is measured by titration), in the middle of a stream if it is a river, or in a bucket placed at the pump's discharge if it is groundwater. It is important not to aerate the water (risk of precipitation of elements in solution, oxygenation)
Transportation and storage
From: Sample handling and Storage Requirements for Water and Wastewater Samples. Standard Operating Procedure − Mackay Regional Council. 2015
Refrigeration or freezing
The sample should be kept at a temperature lower than that during filling. Refrigeration or freezing of samples is only truly effective if it is applied immediately after the collection of the samples. This necessitates the use of cool boxes or refrigerators at the sampling site.
Refrigeration entails the placement of samples in a refrigerator, using crushed ice in a portable cooler or other device to cool the sample to a temperature of between 1°C and 4°C. In most cases, this is sufficient to preserve the sample during the transport to the laboratory and for a relatively short period of time before the analysis. Refrigeration cannot be considered as a means of long-term storage, particularly in the case of waste water samples.
Freezing to a temperature of −20°C allows, in general, an increase in the period of storage. For some analytes, such as nutrients, freezing is the preferred method of preservation. Do not refreeze samples. Sufficient individual portions should be collected in cases where analyses may be conducted at different times or locations. When thawed, samples should be thoroughly mixed and allowed to reach ambient temperature before any measurements are made.
- Quick Guide To Drinking Water Sample Collection United States Environmental Protection Agency